All bit overly touchy, aren't we?  See Merriam-Webster Dictionary.  The abbreviation for Japan/Japanese is "Jap".  And he makes a possible good point, which you are detracting from. I wouldn't consider it a racial slur, but more a way to get his basic thought in the subject line (that's why abbreviations are used).

Do you get as pissed when someone calls us USA folks "Yanks"?  If you think about it, that word has nasty conotations! (Although sometimes our foreign policy makes that word quite apt! LOL).

JMHO, and that of Merriam-Webster

SGE